WASHINGTON — President Obama was once Hawaii’s pothead-in-chief, according to a new biography.

Long before he took briefings from the Joint Chiefs, the prez was a joint-loving member of a high-school dope-smoking club who tooled around Hawaii in a hippie VW van dubbed the “Choomwagon” and blasted Aerosmith tunes as they got wasted, the book says.

Obama and his pals called themselves the “Choom Gang” — local slang for pot-smoking.

Obama even gave a shout-out to the group in his yearbook when he graduated in 1979 from the exclusive Punahou school.

According to David Maraniss’ biography “Barack Obama: The Story,” the future president enacted a liberal weed-smoking policy in the Choomwagon.

“When a joint was making the rounds, he often elbowed his way in, out of turn, shouted, ‘Intercepted!’ and took an extra hit. No one seemed to mind,” Maraniss writes.

Obama also popularized “roof hits,” long before the release of “Fast Times at Ridgemont High,” the 1982 comedy that features a scene in which a bunch of high school stoners tumble out of a smoke-filled van.

“When they were chooming in the car, all the windows had to be rolled up so no smoke blew out and went to waste; when the pot was gone, they tilted their heads back and sucked in the last bit of smoke from the ceiling,” according to the book.

The “Choom Gang” liked to drive up Mount Tantalus on Oahu, where they parked, “turned up their stereos playing Aerosmith, Blue öyster Cult, and Stevie Wonder, lit up some ‘sweet-sticky Hawaiian buds,’ and washed it down with ‘green bottled beer,’ ” Maraniss writes.

They got their pot from a dealer named “Ray,” who worked at the Mama Mia Pizza Parlor and was a bit older than the group, the book said.

“Many years later they learned that he had been killed with a ball-peen hammer by a scorned gay lover,” according to the book.

Obama even thanked his bong buddies — and their dealer — in his high school yearbook: “Thanks Tut, Gramps, Choom Gang, and Ray for all the good times.”

Maraniss noted that, “A hippie drug-dealer made his [Obama’s] acknowledgments; his own mother did not.”